This invention relates generally to a method for determining leakage from a nominally sealed product container. The container, for example, can comprise the case of an electrochemical battery but more generally can comprise any product container containing a liquid component, e.g., electrolyte. A method in accordance with the invention is primarily intended for batch testing multiple product units in the manufacturing process of batteries and the like for detecting leakage sufficiently slow as to be unapparent to the naked eye.
For battery and capacitor manufacturers, it is of utmost importance from a safety and reliability standpoint, that the product case be properly sealed. A variety of commercially available leak testing apparatus and methods exist that attempt to measure leak rates including electronic sensors, helium leak detectors, pH measurement devices, and visual inspection. Electronic sensors typically have detection limits in the parts per ten thousand range and are usually complex and not practical for high volume processing. High-end helium leak detectors have detection limits of 10xe2x88x9210 cc-atm/sec. However, if the leak opening is greater than the helium atom but smaller than the molecules of the leak component of interest, helium easily escapes the product container and the test results in a false negative. Another method measures chemical behavior that can be differentiated along the pH scale. This method is qualitative and limited to small families of chemicals that are pH sensitive. Finally, the traditional method of visual inspection, either by the naked eye or through optical microscope, is magnification and speed limited. For example, the conventional visual inspection method involves placing batteries in a controlled temperature environment for several days or weeks. Signs of leakage would typically show as marks of residual chemical corrosion and the like on the surface of the battery case emanating from the leakage point. In mass production, this method is labor-intensive and quite costly.
The present invention is directed to a method and apparatus suitable for use in high volume production for determining leakage from a nominally sealed product container, e.g., a battery case. More particularly, the invention is directed to a method and apparatus for analytically testing for leakage of a particular component of interest, e.g., a liquid electrolyte comprising a mixture of ethylene carbonate (EC) and methyl-ethyl carbonate (MEC) in a lithium-ion battery.
A method in accordance with the invention first requires that the product container be cleaned to remove measurable traces of the leakage component of interest. The cleaned container is then placed in a sealed vessel containing an extraction medium compatible with the leakage component. The extraction medium should be miscible with the leakage component and readily separable from the leakage component using commercially available separation apparatus, e.g., gas chromatograph. After one or more predetermined time intervals, a sample of the extraction medium is collected from the vessel. The collected sample is then analyzed in a high sensitivity chromatography system to determine the concentration of the leakage component of interest. Leakage rate can then be determined based on the measured concentration and the known time interval. If the leakage rate exceeds a certain threshold, the product is rejected.
In accordance with a preferred embodiment of the invention, a product container which passes the aforedescribed first leakage test stage is preferably subjected to a second stage which tests for gas leakage. This second stage presumes that the product container in fabrication was sealed in the presence of a gas such as helium, argon, nitrogen and the like. In order to test for leakage from the container, the container is placed into a test cavity which is evacuated to a very low negative pressure. Leakage from the container is then monitored over time to detect presence of the gas in which the container was originally sealed, e.g., helium or argon or nitrogen.